With recent advancement of digital technologies, electronic hardware such as portable information devices and home information appliances have been developed to achieve higher functionality. With the achievement of higher functionality of the electronic hardware, development of larger-scale, highly-integrated, and higher-speed nonvolatile memory devices incorporated thereinto have been progressing at a high pace, and the uses thereof have been spreading at a rapid pace.
Among the nonvolatile memory devices, a memory device including nonvolatile resistance variable elements used as memory elements and arranged in matrix is proposed. This memory device is expected to achieve larger scale, higher integration, and higher speed as a three-dimensional memory.
The resistance variable element has a thin layer which is made of a material mostly composed of a metal oxide. Upon application of an electrical pulse to the thin layer, its electrical resistance value switches, and the switched electrical resistance value is retained. By corresponding a high-resistance state and a low-resistance state of the thin layer to binary data “1” and “0,” respectively, for example, the binary data can be stored in the resistance variable element. The current density of the electrical pulse applied to the thin layer of the resistance variable element or the magnitude of the electrical field generated by application of the electrical pulse is set to be large enough to change a physical state of the thin layer but not so large as to destroy the thin layer.
Among the resistance variable elements which are adapted to have binary values, a resistance variable element (so-called unipolar resistance variable element) which switches its resistance value upon application of electrical pulses with the same polarity and different voltages to the element, and a resistance variable element (so-called bipolar resistance variable element) which switches its resistance value upon application of electrical pulses with different polarities to the element, are known. In general, the unipolar resistance variable element requires a longer write time to switch from a low-resistance state to a high-resistance state (so-called reset) than to switch from the high-resistance state to the low-resistance state (so-called set). On the other hand, the bipolar resistance variable element is capable of writing data in a short time in both of the set and the reset.
In the memory device (so-called cross-point memory device) including the plural above-mentioned resistance variable elements respectively arranged at three-dimensional cross sections of plural word lines and plural bit lines which cross each other at a right angle so as not to contact each other, a problem (hereinafter this problem is referred to as “write disturb”) could occur, in which an electrical resistance value of another resistance variable element switches due to a leakage current, when data is written to a certain resistance variable element. For this reason, in forming such a cross-point memory device, a special configuration is required to prevent the write disturb.
The unipolar resistance variable element is capable of switching its resistance in response to electrical pulses with the same polarity. Therefore, the write disturb can be prevented by arranging a unipolar current control element (having a nonlinear voltage-current characteristic having a high-resistance state and a low-resistance state in a voltage range of one voltage polarity), such as a p-n junction diode or a Schottky diode, in series with the resistance variable element.
As a memory device capable of preventing the write disturb, a memory device is disclosed, in which a memory element has a series circuit including a resistance variable element and a Schottky diode (current control element) (see patent document 1, for example).
In this proposed memory device, a leakage current flowing toward a resistance variable element is blocked by the Schottky diode in memory elements other than a memory element to which data should be written (so-called selected memory element). Thus, in the cross-point memory device, the write disturb is prevented. In this proposed memory device, data is written to the resistance variable element by applying electrical pulses with the same polarity to the resistance variable element. Therefore, the Schottky diode connected in series with the resistance variable element does not impede writing of data.
In contrast, in the case of using the bipolar resistance variable element, bipolar electrical pulses are used to write data in the resistance variable element. Therefore, it is necessary to arrange a bipolar current control element (having a nonlinear voltage-current characteristic having a high-resistance state and a low-resistance state in a voltage range of positive and negative polarities, respectively) in series with the resistance variable element. As the element having such a characteristic, two-terminal elements such as a MIM (Metal-Insulator-Metal) diode, a MSM (Metal-Semiconductor-Metal) diode and a varistor are known.
FIG. 17 is a view schematically showing a current-voltage characteristic of a current control element, in which FIG. 17(a) is a view of a voltage-current characteristic of a bipolar current control element such as the MIM diode, the MSM diode, or the varistor, and FIG. 17(b) is a view of a voltage-current characteristic of the Schottky diode.
As shown in FIG. 17(b), the Schottky diode exhibits a non-linear electrical resistance characteristic but exhibits a current-voltage characteristic which is far from being symmetric with respect to the polarity of the applied voltage.
On the other hand, as shown in FIG. 17(a), the two-terminal element such as the MIM diode, the MSM diode, or the varistor exhibits a non-linear electrical resistance characteristic and exhibits a current-voltage characteristic which is substantially symmetric with respect to the polarity of the applied voltage. To be specific, a change in the current with respect to a positive voltage applied and a change in the current with respect to a negative voltage applied are substantially symmetric with respect to an origin 0. In these two-terminal elements, an electrical resistance is very high in a range (i.e., range C) in which the applied voltage is not larger than a first critical voltage (the lower limit voltage in a range A) and is not smaller than a second critical voltage (the upper limit voltage in a range B), whereas the electrical resistance drastically decreases in a range in which the applied voltage is larger than the first critical voltage or smaller than the second critical voltage. In other words, these two-terminal elements exhibit a non-linear electrical resistance characteristic in which a large current flows when the applied voltage is larger than the first critical voltage or is smaller than the second critical voltage.
Therefore, by using these two terminal elements as the bipolar current control elements, the write disturb can be avoided in the cross-point memory device incorporating the bipolar resistance variable elements which are capable of a high-speed operation in both of the set and the reset.
To allow the resistance variable element to switch to a high-resistance state or to a low-resistance state by applying an electrical pulse to the resistance variable element to change the electrical resistance value, when writing data to the resistance variable element in the resistance variable memory device, in most situations, it is necessary to feed a large current to the resistance variable element, although it depends on the material or structure of the resistance variable element, etc. For example, it is disclosed that a current with a density of 30000A/cm2 or higher is flowed using the varistor when writing data to the resistance variable element, in the operation of the memory device including the resistance variable elements (e.g., patent document 2).
Tungsten has three states which are called α-state, β-state and amorphous state. Patent document 3 discloses β-tungsten as tungsten used for a wire material.